1. Field of the Invention
The present invention relates to a tint block image generation program and a tint block image generation device, and more particularly to a program and device for generating tint block image data to be printed on a print medium, which has an effect to inhibit forgery by copying an original print medium on which a tint block image is printed based on the tint block data.
2. Description of the Related Art
The tint block is combined with the original image as background, and allows distinguishing whether the print document is the original or the copy. Characters or images in the tint block can hardly be identified in the original, but if copied, the characters or images in the tint block emerge. Using this, the original and the copy can easily be distinguished. Also the characters or images in the tint block emerge in copying, so if an original is generated combining with the tint block, an attempt to copy the original is psychologically discouraged.
The tint block is disclosed in Japanese Patent Application Laid-Open No. 2005-151456, and details follow according to this description.
Generally a tint block is comprised of two areas: a “latent image portion” where dots printed in the original remain or decrease little by copying, and a “background portion” where dots printed in the original are lost or greatly decreased by copying. In other words, in the latent image portion, density changes little by copying, and the original image is reproduced as is, and in the background portion, density changes considerably by copying, and the original image disappears. The characters or images of the tint block are generated by these two areas, and the characters and images of the tint block are called the “latent image”.
The densities of the latent image portion and the background portion are roughly the same, and in the original state, it is visually difficult to find such characters or images as “COPIED” of Japanese character are concealed in the tint block, but at the micro level, the background portion and latent image portion have different characteristics. When the tint block is copied, a density difference is generated between the latent image portion and the background portion, because of the difference of the respective change of density, which makes it easier to discern the characters or images of the tint block created by these two areas.
The latent image portion is comprised of clustered dots so that dots can be easily read when copying (scanning by copying), whereas the background portion is comprised of dispersed dots so that dots cannot be easily read when copying. By this, dots tend to remain in the latent image after copying, and dots tend to disappear in the background portion more easily than the latent image portion. Clustered dots or dispersed dots can be implemented by half tone processing using a different number of lines of half tone dots. In other words, half tone dots of which screen ruling is low are used to obtain a clustered dot arrangement, and half tone dots of which screen ruling is high are used to obtain a dispersed dot arrangement.
Generally a copier has a limitation in image reproducing capability, which depends on the input resolution in a step of reading the micro dots of a copy target original by a scanner, and the output resolution in a step of reproducing micro dots, read by the scanner, using a print engine. Therefore if isolated micro dots exist in the original, exceeding the limitation of the image reproducing capability of the copier, the micro dots cannot be perfectly reproduced in a copy, and the portions of the isolated micro dots disappear. In other words, if the background portion of the tint block is created so as to exceed the limitation of the dots that the copier can reproduce, then large dots (clustered dots) in the tint block can be reproduced by copying, but small dots (dispersed dots) cannot be reproduced by copying, and a concealed latent image appears in the copy. Even if the dispersed dots in the background portion do not disappear completely by copying, a density difference is generated between the background portion and the latent image portion after copying if the degree of loss of dots is high, compared with the clustered dots in the latent image portion, then a concealed latent image appears in the copy.
In the tint block, a technology called “camouflage” is used to make it more difficult to discern characters or images concealed as a latent image. This camouflage technology is a method for arranging patterns, of which density is different from the latent image portion and the background portion, in the entire tint block image, and in a macro view, the camouflage patterns, of which density is different from the latent image portion and the background portion, standout, making the latent image even more obscure. In other words, the contrast of the camouflage patterns is high, and the contrast of the latent image portion and the background portion is smaller than this, so the latent image is more effectively concealed because of optical illusion. Also the camouflage pattern can give a decorative impression on printed matter, and allows creating an artistically designed tint block. Generally a camouflage pattern is created in binary, and the camouflage pattern is formed by not generating dots of the tint block in an area corresponding to the camouflage pattern. The camouflage pattern with two grayscales is disclosed in Japanese Patent Application Laid-Open No. H04-170569. The above is an overview of the tint block.
FIG. 1 shows an example of a latent image of a tint block and a camouflage pattern. In a latent image mask pattern 10 of the Japanese character “COPY”, the black portion corresponds to the latent image portion LI of the tint block, and the white portion corresponds to the background portion BI of the tint block, for example, as the enlarged view 10X shows. In the camouflage pattern 12, on the other hand, the black portion CAM becomes an area where the dots of the tint block are not formed, and the white portion becomes an area where dots of the tint block are formed, for example, as the enlarged view 12X shows.
FIG. 2 is a diagram depicting an example of an original in which a tint block is printed. In the tint block 14, a latent image portion LI and a background portion BI are formed based on the latent image mask pattern 10 in FIG. 1. The latent image portion LI is formed by dots with low screen ruling (53 lpi) based on a clustered dot dither method, and the background portion BI is formed of dots with high screen ruling (212 lpi) based on the dispersed dot dither method. As the enlarged tint block 14X shows, the entire tint block has a predetermined output density, but the dots in the latent image portion LI are large dots formed by a screen with low screen ruling, and the dots in the background portion BI are small dots formed by a screen with high screen ruling.
In the tint block 16, the latent image portion LI and the background portion BI are formed, excluding a black area CAM of the camouflage pattern, based on the latent image mask pattern 10 and the camouflage pattern 12 in FIG. 1. As the enlarged tint block 16X shows, the entire tint block has a predetermined output density, where dots are not formed in the area CAM of the camouflage pattern, and in another area, the latent image portion LI formed by large dots and the background portion BI formed by micro dots are formed just like FIG. 1. Since the contrast of the camouflage pattern is high, the latent image (the Japanese character “COPY”), comprised of the latent image portion LI and the background portion BI, of which contrast is low, does not stand out.
In the original of the forgery inhibited tint block in FIG. 2, the output density of the latent image portion LI and the background portion BI are the same, whereby the latent image of the Japanese character “COPY” formed by these portions is concealed. This is referred to as the “concealment capability for a latent image in the original is high”.
FIG. 3 is a diagram depicting an example of a copy of the tint block. The copy 18 is created via a scanning step and dot generation step (step of printing the print media based on the scan data generated in the scanning step) by copying, and as the enlarged view 18X shows, large dots in the latent image portion LI are hardly lost, but many micro dots in the background portion BI are lost. As a result, in the copy 18, the output density of the latent image LI hardly drop, but the output density of the background portion BI drop considerably, and the latent image of the Japanese character “COPY” emerges. In other words, the latent image of the copy is more easily identified.
The copy 20 is the same as the copy 18, except for the area CAM of the camouflage pattern. The contrast of the camouflage pattern drops because of the drop in the output density of the background portion BI, and the latent image COPY emerges.
FIG. 4 are diagrams further enlarging the enlarged view of the original in FIG. 2 and the enlarged view in the copy in FIG. 3. In the original shown in (a), the latent image portion LI is formed by dots (halftones), with low screen ruling and a large area, and the background portion BI is formed by micro dots with high screen ruling. No dots are formed in a black portion CAM of the camouflage pattern. In the copy (b), on the other hand, the size of the large dots (halftones) in the latent image portion LI do not change much, but a considerable number of micro dots in the background portion BI are lost. As a result, in the copy, the output density of the latent image portion LI hardly drops, while the output density of the background portion BI drops considerably where the latent image “COPY” of the tint block emerges clearly.